Stressors

Changes in hydrology

Climate-induced changes in precipitation, winter conditions, and extreme storm events have increased base and average stream and river flows in many parts of New England. Land use practices, water withdrawals for human use, and development are also influencing hydrological conditions of water bodies and aquifers. Intensification of the hydrologic cycle (evaporation, condensation, precipitation, etc.) due to climate change and extreme precipitation events can increase the delivery of nutrients and pollutants to downstream and coastal habitats. This has important implications for food-web structure and ecosystem function, such as making poor water quality events (e.g., excessive nutrient loading) and the incidence of waterborne disease more likely. Hydrological conditions and the ability of aquatic species to travel from stream to stream (i.e., aquatic connectivity) are important for aquatic fish and wildlife survival. Human structures that intersect with aquatic systems, such as dams, culverts, and road crossings, are potential barriers to fish and wildlife movement.

Stream and River Flows
Climate change directly impacts stream and river flows through changes in precipitation, temperature, and evapotranspiration? (the sum of evaporation and the movement and exchange of water among plants, land, waterbodies, and the atmosphere). However, impacts will vary among basins based on:

Annual average river and stream flow rates have increased during the last part of the 20th century throughout the Northeast region, even though peak flow rates have remained constant. “Low flows” play a crucial role in maintaining aquatic ecosystems as they provide a base level of flow (i.e., sustained flows composed of normal inputs from groundwater and precipitation) and help control stream water temperatures, particularly during summer. What constitutes a low or minimum flow varies by study, location, and season, but is generally the lowest flow for that site (excluding zero level flows or no water) over a defined time period (e.g., a week). Precipitation increases due to climate change do not appear to be changing low flows consistently across all seasons; for example, there is evidence that more precipitation during late summer/early fall has increased low flows later in the fall.

Seasonal and Decadal Influences on Flows
Warmer winter and early spring temperatures are leading to earlier snowmelt, more rain-on-snow episodes, and the breakup of winter ice on regional water bodies. Because of this, the timing and magnitude of spring flows is shifting, which affects fish and other aquatic or wetland species that depend on flows and aquatic corridors for migration, spawning, development of early life stages, or other vital life cycle transitions. Winter-spring peak flows occur approximately 6 days earlier compared to a century ago in Massachusetts and this trend is projected to continue during the 21st century. In addition, a shift toward higher winter flows and lower spring flows has been documented for the Connecticut River Basin as well as just to the north in New Hampshire at the Hubbard Brook Long Term Ecological Research Station using climate-driven streamflow simulations.

Large shifts in the magnitude of maximum annual streamflows have also been documented in many stream and river basins in New England. These shifts are associated with decadal climatic cycles such as the North Atlantic Oscillation, a natural, somewhat random phenomenon that produces changes in atmospheric pressure and sea level heights. These shifts are abrupt and short-lived, lasting no longer than a year. Larger peak seasonal flows and velocities contribute to increases in river scour magnitude and frequency, and affect egg burial depths, displacement, and survival of some fish and animals, such as freshwater mussels.

Groundwater Influences on Streams and Rivers
Groundwater inputs into streams and rivers are also important regulators of temperature and flow. Groundwater can contribute to coldwater refugia for fish and other cold-adapted aquatic organisms, especially during summer and low flow periods. Changes due to winter warming and the timing of spring melting are expected to reduce groundwater reserves during subsequent seasons. In addition, drought conditions affect water supply to freshwater systems and impact their chemistry and temperature; the magnitude of these effects is partially dependent on whether the system is primarily fed by groundwater or surface waters (e.g., stream flows and rainfall). Alternatively, recent studies in New England suggest groundwater table levels are rising, potentially increasing the risk for flooding.

Lakes and Pond Levels
Lakes are indicators of climate change in part because they reflect climatic warming and are directly influenced by changes to the surrounding terrestrial landscape. Lakes are warming at rates similar to air temperatures on a global scale. Responses of lakes to climate change are complex and variable, and often depend on individual basin characteristics, such as:

Intensification of the hydrological cycle is affecting the transport of nutrients and pollutants to downstream and coastal habitats, impacting fish movements, and restructuring food-webs and downstream habitats. Image credit: Jane Thomas

Water depth

Surface water influences

Groundwater influences

The local landscape

Nearby vegetation and forest coverage will affect how exposed or sheltered small and large water bodies are to wind, which influences the vertical distribution of water temperatures (i.e., temperatures at varying depths). Vegetation can also provide protection from erosion and the influx of nutrients and pollutants to lakes and ponds.

Climate Induced Changes in Nutrient Levels to Lakes and Ponds
Increased precipitation and extreme events due to climate change are increasing the delivery of nutrients to lakes from terrestrial runoff, which results in degraded water clarity and increased stratification (i.e., increasing the difference between warmer surface waters and colder bottom waters). These chemical and physical changes have important impacts on photosynthesis and aquatic food-webs. Lakes and ponds are also impacted by atmospheric deposition of sulfur and nitrogen oxides, which change the chemistry of waters through acidification.

Acidification (or deacidification) affects the levels of dissolved organic carbon (DOC) as well as water temperature and clarity. DOC is generated from the decomposition of living matter, such as vegetation and animals, and represents the primary form of carbon transported and exchanged within soils and aquatic ecosystems. DOC is naturally higher in certain types of waterbodies, such as those with surrounding wetlands or peatlands, but can also be the result of pollution. Too much DOC in a system can result from high nutrient loading that overstimulates primary production, for example from an algae bloom. This leads to increased rates of decomposition and ultimately hypoxic (low oxygen) conditions that can be deadly to other organisms.

Seasonal Changes in Ice and Water Levels
Warmer winter air temperatures are impacting ice cover and duration over lakes and ponds in Massachusetts. Ice on lakes and ponds has been breaking up earlier by a week or more in New England. Earlier melting of snow and ice during winter and spring are altering the seasonal patterns of water levels, or the hydroperiod, in water bodies and surrounding wetlands. Future projections of climate change are expected to reduce the hydroperiod of ephemeral ponds and wetlands (e.g., vernal pools) due to temperature effects on evaporation rates, and changes in seasonal precipitation rates. However, trends in evapotranspiration? rates vary widely across studies and are generally inconclusive. No trend has been seen in evapotranspiration measurements in the Connecticut River Basin since 1950.

3. Brooks, R.T. 2009. Potential impacts of global climate change on the hydrology and ecology of ephemeral freshwater systems of the forests of the northeastern United States. Climate Change 95:469-483.

15. Parr, D. and G. Wang. 2014. Hydrological changes in the U.S. Northeast using the Connecticut River Basin as a case study: Part 1. Modeling and analysis of the past. Global and Planetary Change 122:208-222.